This invention relates to a method for making improved tubular metallic shafts for golf clubs and other sporting implements.
As is commonly known, a golf shaft undergoes a significant stress during a golf swing at the portion of the shaft where the club head is attached. Typically, this portion of the shaft is of the narrowest diameter with respect to the remainder of the shaft since most golf shafts have a tapered configuration. Thus, this portion is especially susceptible to deformation if excessive force is used in hitting a golf ball or, in the alternative, a mis-hit occurs and the club head hits the ground.
The most convenient way of eliminating such a problem area on the shaft would be to increase its diameter to a value closer to the diameter of the rest of the shaft. Such a remedy is highly undesirable, however, because the weight distribution and moment of inertia inherent in a narrowing diameter or tapering shaft is necessary for execution of the most effective golf swing. More particularly, a tapered shaft is necessary in order to provide the proper "flex" and "flex point" of the shaft for an effective stroke. Both the "flex" and the "flex point" are determined according to the tapering nature of the shaft.
Consequently, various tip configurations have been used to reinforce this segment of the shaft while retaining its narrowing characteristic, the most common perhaps being the incorporation of a reinforcing metal insert. Such an insert, however, adds undesired weight to the shaft and also necessitates some kind of retaining feature to hold it in place. Such a retaining feature may include the use of a retaining pin or a special mechanical joining operation.
Methods for making shafts with varying wall thickness are contemplated in the prior art. For example, U.S. Pat. No. 2,095,563 to Cowdery discloses a method of making a golf shaft wherein a tip portion has a wall thickness larger than that of the remaining portion of the shaft. However, the increased wall thickness is achieved by an operation which usually fails to give a constant wall thickness along the shank portion of the shaft thus adversely affecting weight distribution.
U.S. Pat. No. 2,240,456 to Darner and U.S. Pat. No. 4,616,500 to Alexoff show methods for providing varying wall thickness on a shaft with a constant outer diameter.
U.S. Pat. No. 3,292,414 to Goeke shows a method that provides a shaft with a tapered end, the tapered end having internal corrugations for strenghthening.
U.S. Pat. No. 3,841,130 to Scott, Jr. et al. shows a baseball bat with a tapered, constant-thickness wall.